Temperature responsive safety devices for munitions

ABSTRACT

The invention comprises devices for mitigating the explosive reaction of a munition when it is subject to an external thermal hazard threat. The devices are based on the use of shape memory alloys. In one arrangement there is device which consists of a connector that is at least in part formed from a shape memory alloy, which typically undergoes large dimensional changes when heated or cooled through a particular transition temperature range. The connector in this invention is designed to form a locking engagement, between two components of a munitions casing at one temperature, but when subjected to external heating through the transition temperature range will deform to allow the connector to disengage and thus release the two joined components, allowing any build up of pressure to be released quickly. Advantageously if the co-operative parts of the connector and components are threaded portions, then the locking engagement will be capable of being dismantled during normal servicing of the munition. The co-operative parts of the connector may be integral with the components to be connected. In another arrangement the device is an annulus and is located around a munitions casing such that upon heating through its transition temperature range will cause the annulus to contract, thereby rupturing the munitions casing, allowing any build up of pressure to be released quickly.

The present invention relates to the use of shape memory alloys in theconstruction of devices, which are designed to disengage two componentson being heated to a predetermined temperature. A particular applicationfor the device is to a munitions casing in order to help avoid or atleast to mitigate an explosive reaction when such munitions areinadvertently exposed to fire or some other source of heat.

By the term “munitions” as used hereinafter is meant a bomb, warhead orrocket motor or any similar device which contains a gun propellant, arocket propellant or an explosive or other energetic material housedwithin a casing.

The present invention is concerned particularly with the use of shapememory alloys (SMAs) as providing means for mitigating against theviolent explosive reaction of a munition when it is heated to theignition temperature of the energetic material. The most extremecondition occurs when the rate of heating is very slow, the so-called“slow cook-off” condition. Under these circumstances, the whole munitionreaches an almost uniform temperature so that the casing surrounding theenergetic material is unlikely to lose very much strength before thepoint at which the energetic material finally ignites. At this pointthere is a rapid pressure build-up and a high order explosion or even adetonation occurs. Faster heating, which occurs for example when themunition is exposed to a fuel fire (a so-called “fast cook-off”condition) is less hazardous and easier to counter. In this situation,because the flow of heat is from the outside of the munition to theinside, the casing will reach a higher temperature than the energeticmaterial and so will weaken before the energetic material ignites. It ispossible to enhance this effect by choice of case materials and by theuse of thermal insulation (which is usually needed anyway) between thecase and the energetic material. Although the present invention isconcerned with mitigating both fast and slow cook-off, the emphasis ison the latter because of the lack of alternative measures for meetingthis situation.

There have been a number of disasters over the last 40 years, involvingships, magazines and weapon storage depots in which much loss of lifeand military equipment have been incurred. Alarmingly many of them haveoccurred during peace time, and, of those that have occurred in wartime,many have not been the result of enemy action

Slow cook-off events have typically occurred where there is a fire in acompartment next to a magazine, which burns for many hours with theresult that the magazine heats up slowly and all the explosive storeswithin it increase in temperature very slowly and uniformly. Therefore,when the first particle of energetic material reaches its spontaneousignition temperature (T of I), probably in the range 125° C. to 200° C.,the remainder is also on the verge of igniting. Furthermore, at thattemperature the munition casings would retain nearly all of theirstrength, particularly if they were made of steel. The result can be ahigh order explosion that can, for example, destroy a ship. Two famousexamples of disasters initiated by fires are HMS Sheffield in theFalklands War and the USS Forrestal in the Vietnam War, both of whichresulted in large casualties and loss of platforms and systems andmunitions.

As a result of these and other incidents, the subject of InsensitiveMunitions (IM) has become an important one in the design, procurement,storage and deployment of any weapons system that employs propellants orexplosives, that is most weapons. There is now a general requirement todesign main charges, booster charges, explosive trains, rocket motorsand gun propellant charges such that when exposed to a disruptive threatthey respond as benignly as possible. Therefore, ideally they shouldgive rise to a burning reaction, rather than a high order explosiveevent or a detonation. In this way it is hoped to avoid the generationof a shockwave or of damaging fragments that would adversely affectother weapons stored in the proximity. By so doing, the hope is thatfratricidal events or “chain reactions” can be avoided.

One way to achieve such IM status is to develop propellants andexplosives that are relatively insensitive to shock and fragment attackand much work has been carried out on this over the last 25 years, withnew generations of energetic materials emerging, albeit slowly.

Another approach is to design the hardware items, i.e. rocket motor orwarhead casing, so that when they are attacked they break open readilyand do not allow a rapid pressure build-up that might lead to adetonation or high order explosive event. To some extent, it isdifficult to reconcile this requirement with the need to withstand roughhandling. Nevertheless some satisfactory compromise solutions have beenachieved.

There are several standard IM tests, of which three of the most commonlyused are:

-   -   Bullet or fragment impact    -   Fuel fire (so-called fast cook-off)    -   Slow cook-off (SCO)

These tests are designed to replicate the common threats that may causepremature, unwanted, detonation of munitions. Methods have been devisedfor combating the first two of these threats, but mitigating againstslow cook-off has remained an intractable problem.

Previously a number of methods have been suggested for attempting tomitigate against premature detonation of munitions under slow cook-offconditions. These have included:

-   -   1. The use of line cutting charges on the outside surface of the        case, and pointing inwards. Used in association with an        appropriate sensor, it can be arranged for such a charge to cut        a slit in the case just before the propellant ignites.    -   2. Thermite blocks have also been used to achieve a similar        result by burning a hole in the case.

3. Low melting alloys or polymer compositions have been considered as ameans of greatly reducing the strength of a joint when subject to heat.

None of these methods has proved particularly successful whether appliedto rocket motor cases or to other types of munition. The first twomethods are considered as active mitigation methods, which involve theuse of additional energetic materials on the body of the weapon, whichcan introduce a further set of hazards making them an unattractivesolution. The third method is referred to as passive mitigation.However, the problem encountered with this type of passive mitigation,using low melting materials, is trying to achieve sufficient strengthunder normal firing conditions. At the same time it is necessary toensure that most of the strength has been lost at the lowest possiblepropellant ignition temperature. For a double base propellant thistemperature can be as low as 125° C. An alternative method, by which alow melting point material is used as a fusible plug, is inadequatebecause it cannot be used to create a large enough aperture for thegaseous products from the propellant or explosive to vent sufficientlyquickly.

Shape memory alloys are metal alloys that undergo large dimensionalchanges when heated or cooled through a particular transitiontemperature range. Shape memory alloys exhibit two distinct crystalstructures or phases below and above the transition and the mechanicalproperties of the alloy are different in the two phases. Therefore, uponheating or cooling the alloy, a transition temperature range is reachedover which range the crystal phase changes and the alloy will adopt theproperties of the new crystal phase. In general, the “memory” isimparted to the SMA by deforming it, usually in the lower temperaturestate. Therefore a ring which is intended to expand on heating throughits transition temperature range would previously have its memoryimparted at a lower temperature by compressing it radially. Whereas, aring intended to shrink on heating would have the memory imparted bystretching. An SMA material is said to exhibit one way memory if theshape change achieved by plastic deformation at a lower temperature isannulled on heating and the deformed shape is not restored on subsequentcooling. By contrast SMA materials which can be made to alternatebetween a low temperature shape and a high temperature shape throughouta number of heating and cooling cycles are said to exhibit the two-wayshape memory. Both types of shape recoveries are possible in most of theSMAs. However the extent of reversible shape recovery associated withtwo-way shape memory in any SMA is usually less than that associatedwith one-way memory. In general, though, unlike low melting point metalalloys, which are mechanically weak, SMAs have mechanical propertiesthat are comparable with those of engineering materials such as lightalloys and steels and are therefore ideally suited to high stress andstrain applications. The transition temperature for the shape change canbe selected by the appropriate choice of composition of the SMA.

The one way recovery strain achievable is in the range 2% to 6% in Ti—Nibased SMAs and in the range 1% to 4% in Cu—Al based SMAs. In general,the highest recovery strains are achievable in rings or tubes to whichthe memory is imparted by stretching in a radial direction and whichthen shrink to their original dimensions on heating. In the reversemode, where the memory is imparted by compression and the componentexpands on heating, the effect is somewhat smaller, but neverthelesslarge enough to be usable.

A tube manufactured from a shape memory alloy which is designed toexpand radially upon heating will usually contract in length at the sametime, as the overall volume of the shape memory alloy remainssubstantially constant. Likewise, if the tube is designed to contractradially, this will lead to a concomitant expansion along the axis. Forthe purposes of the current invention, it is also significant that manyshape memory alloys will generate high recovery strains on activation,even when their movement is opposed by large resistive forces.

Such tubes can be manufactured by machining from rod, forging orextrusion, alternatively; for large diameter tubes it may be moreconvenient to select SMA alloy sheets of appropriate thickness, wrapthem around suitable mandrels to achieve cylindrical shapes and weld thejoints to produce SMA tubes. In the latter case there may be some lossof SMA function at the weld interface, but the remaining SMA will givethe required expansion or contraction on heating.

U.S. Pat. No. 6,321,656 discloses the use of shape memory alloys tomitigate against slow cook-off in relation to rocket motors. The patentdescribes three embodiments of the invention as applied to a rocketmotor case, which is in two sections. A first section has a small numberof prongs each with a small lug at its tip and the second section has anequal number of recesses for location of the lugs. When the two sectionsare brought together in an end to end manner the lugs engage with therespective recesses by virtue of the prongs on the first section beingbiased so as to cause each associated lug to lock with its respectiverecess in the second section. In a first embodiment of the invention, ashape memory alloy ring, which is of an alloy composition such that uponheating it will contract, is located tightly around the prongs. Uponheating, in a thermal hazard incident, the shape memory alloy ringcontracts, pushing the prongs inwards and therefore causing the lugs tomove out of their respective recesses allowing the two sections of themotor case to disengage and so to vent any built up pressure. In asecond embodiment, the shape memory alloy ring is placed on the insideof the prongs on the first section, and is expanded so as to force theprongs into engagement with their corresponding recesses. On heating thering retracts to its annealed size thereby allowing the prongs on theinner section to move inwards away from engagement with the respectiverecesses in the outer section. In the third embodiment, the firstsection is slightly modified to allow the location of two shape memoryalloy rings, one around the outside and one on the inside of the prongedsection, thus providing the combined effects of the first and secondembodiments, such that upon heating both rings contract inwards, to givethe same overall effect.

However, the arrangement shown in the U.S. patent suffers from thedisadvantages that once the ring or rings have been put into position,they cannot be easily removed without heating the device. It is commonpractice for munitions to be regularly serviced and monitored duringtheir service life and so a non-reversible system such as this would notbe an ideal solution. Another disadvantage is that the pronged sectionproduces an internal projection into the volume where the propellant islocated. This results in difficulties for loading the propellant when incartridge form into the rocket casing and means that the propellantwould most likely have to be melt cast. A further disadvantage of thearrangement shown in this U.S. patent is that the shape memory alloy hasto be heat treated to enable the connection means to be installed. Inaddition, as the whole of the axial load arising from the pressurisationof the case has to be carried through the prongs and lugs, thearrangement is structurally inefficient. Finally, the shape memory alloyring in this arrangement is not an integral part of the connectionsystem, thus adding to the complexity of the arrangement and hence thecost of manufacture.

Accordingly it is an object of the present invention to provide anarrangement where the casing of a munition that might be subject to aslow cook off situation is caused to disrupt so as to avoid an unwanteddetonation of the munition, but whereby the arrangement does not preventroutine disconnection or disassembly of the rocket casing. A furtherobject is to provide a means of disruption which is an integral part ofthe connection for a munition casing making construction simpler and thecasing easier and cheaper to manufacture.

Although this invention is primarily concerned with means for mitigatingthe effect of slow cook off in relation to munitions it is alsorecognised that connectors according to the invention may be appropriatefor use in other situations. One such area is for the connection ofpipes or containers involved in the carrying or storage of fluids suchas natural gas. In the event of a heating hazard the gas could becomehighly pressurised, which could cause an explosion. However, the(controlled) release of such a fluid would prevent a violent explosion.The connector in the invention should not be seen however to be limitedto use in conjunction with flammable or combustible fluids as anypressurised fluid can present a hazard. Normally the use of such aconnector would be in conjunction with other safety mechanisms.

A further use for these connectors would be for the joining and easyrelease of structural components such as pipes or as for example thoseused in the construction of oil rigs and which need to be dismantled atthe end of their useful life. The underwater support columns of oil-rigsare sometimes cut with explosive charges, but this has adverse effectson marine life. However if these columns were provided with connectorsaccording to the invention, then, at the end of their service life theconnectors could be heated (e.g. by a thermal jacket), which would allowthe structure to be released and relocated. This could be accomplishedwithout the expense and environmental danger involved in the use of highexplosives. Similar arrangements might be contemplated for dismantlingof other structures which are difficult and possibly hazardous toaccess, such as nuclear power stations or chemical manufacturing plants.However, in all these cases consideration would have to be given tosituations in which the structures may experience severe temperaturesi.e. in a fire hazard situation. Under these circumstances a temperatureresponsive connector activated by heating would only be appropriate ifit could be satisfactorily insulated as otherwise the integrity of thestructure might be compromised. An alternative approach to this would beto employ a temperature responsive connector that was induced todisengage by cooling it to a temperature that could never be experiencedin normal service (e.g. −50° C.).

According to a first aspect of the present invention therefore, there isprovided a connection means for joining together separate components toform a unified body wherein locking engagement can be provided betweenan integral operative part of said connection means and an integralco-operative part of at least one of said components wherein either orboth of the operative and co-operative parts is or are made of a shapememory alloy which occupies a first configuration at a first temperatureand undergoes a change of shape when brought to a second temperature toafford a second configuration, said operative and co-operative partsproviding locking engagement at the first temperature and allowingrelease from said locking engagement at the second temperature.

Typically, the operative part of the connection means will comprise acompression fitting, a snap-type of fitting or will involve the use ofthreaded portions, co-operating with appropriate portions on one or moreof the components. The choice of connection means would be dependent onthe nature of the two components to be joined and the nature of thesituation which the connector is intended to cope with, also whether ornot it was desired that the connections should be reversible. The partsmade from a shape metal alloy may be pretreated if desired in order toimpart a shape memory to the material.

The connection means may form a separate structural and load bearingpart between the two components or may form an integral part of eitherone or both of the components in which said component or components iseither wholly formed of a shape memory alloy or has a shape memory alloyinsert which forms at least the operative part of the connection means.Furthermore the co-operative parts may both be formed from SMAs whereinone part is designed to expand upon heating and the other part isdesigned to contract upon heating, therefore affording an increaseddegree of disengagement. The connection means may be arranged to beeither permanent or reversible such that it can be unfastened withoutbeing subjected to heat or by cutting or otherwise damaging any of theoriginal components or the connection means, where this is a separateentity. It may readily be appreciated that the connection means maypossess more than two operative parts, such as a multi-adapter(T-junction connector), in which the connector and components to bejoined would possess mutually co-operating coupling locking means.

The separate components may comprise two or more parts of a munitionscasing, particularly a rocket motor casing, but may alternativelycomprise two or more pipes or columns, which are to be joined togetherbut where it may be desired to achieve the rapid disconnection of thetwo sections when subjected to a thermal stimulus. In one scenario thestimulus may be from an external hazard such as a fire, or secondly thestimulus may be controlled heating to induce failure of the connectionmeans to allow the easy disassembly of a structure. Advantageously suchfailure can be effected at a remote location such as at a depthunderwater or in a hazardous environment such as in a nuclear reactor orin space.

In the context of the present invention the first temperature is atemperature within the range in which the alloy possesses one phasestructure and the second temperature is a temperature within the rangein which the alloy possesses a different phase structure. The transitiontemperature for a change in crystal phase (and hence shape) thereforelies between the first and second temperatures.

In the connection means according to the current invention the SMA usedwill typically be selected from Cu—Al alloys, Cu—Al—Zn, Cu—Al—Ni,Cu—Zn—Al—Mn, Cu—Ni—Al—Zn—Mn or Ti—Ni alloys. Other elements may be addedto Ti—Ni to adjust the transition temperature or achieve bettermechanical properties. These include Nb or Hf in the range of less than10% and Cr, Fe, or Ce in the range of less than 2%. For the purposes ofslow cook-off mitigation, the transition temperature must be higher thanthe highest temperature incurred in normal service, which may typicallybe between 50° C. and 110° C., depending on the storage and serviceconditions, but below the lowest temperature at which slow cook-off canoccur. This cook-off temperature can be as low as 125° C. for someclasses of propellant but well over 200° C. for some pyrotechniccompositions.

Where the connection means comprises a separate load bearing item notintegral with either or both of the components to be joined, it maycomprise two or more parts, wherein one or more recessed regions,located either internally or externally on the components, can be usedto align and locate with the connection means. In this case theconnection means has respectively one or more complementary external orinternal projections, which when brought into the correct alignment withthe two components will engage with the recesses therein so as to lockthe parts together. Clearly the alternative configuration is possible,with the projections located on the components to be joined and thecomplementary recessed regions formed in the connection means. Othercombinations and arrangements of this type will be readily appreciatedby the skilled person and are to be understood as coming within thescope of the invention. The projections can take the form of anyprotrusion such as a tongue, hooked latch, lug, flange or male threadand the complementary recessed region may, for example, be a pocket,channel, groove or female thread.

In a preferred arrangement where the components to be joined are hollowcylinders, the connection means comprises a separate load bearing membercomprising two or more parts and having two internal and/or externalthreaded portions, arranged to interact with complementary threadedportions on each of the components to form the unified body, such as amunitions casing. The threaded portions at least of the connection meansare made from a shape memory alloy which when subject to heating willdeform causing the threaded portion of the connection means to contractor expand radially (depending on whether the connection means is locatedinside or outside the component) and hence to bring about simpledisengagement of the thread. Alternatively the disengagement may rely onthe concomitant expansion or contraction of the SMA threads in adirection parallel to the axis where the relative movement between theSMA and non-SMA threads causes sufficient damage to the threadedportions as to bring about their disengagement. In practice it is likelythat the disengagement of the two co-operative parts will be afforded bya combination of these two processes taking place. For the purposes ofmitigating a cook-off event it is not necessary to completely disengagethe threads. Thus, if radial disengagement occurred to substantiallyhalf a thread depth, this would be sufficient as the egress of the gasesproduced would push the male threaded section to one side relative tothe female thread. Therefore there would be full disengagement aroundpart of the periphery of the joint, which would be sufficient to destroyits structural integrity.

In a further variant, both co-operative parts of the connection meansmay be formed from SMAs and be arranged such that, upon heating orcooling as the case may be, one of the threads expands radially and theother contracts radially, to more readily afford separation of the two.

The invention is primarily concerned with slow cook-off mitigation andcan be used in conjunction with any container for any energetic materialsuch as a bomb or shell containing high explosive, a torpedo or missilecontaining propellant or a pyrotechnic device. Therefore, it hasparticular application to rocket motors or propellant filled munitions.

In the case of rocket motor casings, during normal operation of a rocketmotor, the temperature responsive connector of the invention must havesufficient structural integrity to withstand the internal pressuregenerated by the burning propellant. At the same time it must besufficiently well insulated from the hot gases to remain below itstransition temperature throughout propellant burn. Normally a rocketmotor has internal insulation to ensure that the case remainssufficiently cool to perform its structural role. If a temperatureresponsive connector is used, some internal insulation may be requiredthat is additional to the amount that would otherwise be needed.Likewise, if the rocket motor is part of a high-speed missile that issubjected to aerodynamic heating, additional external insulation may beneeded to prevent activation of the connector. With the connection meansof this invention present, having a transition temperature which issubstantially lower than the temperature of ignition of the energeticmaterial, the shape memory alloy will adopt its second configurationunder slow cook off conditions before the temperature of ignition isreached, thus allowing the connection means to deform and the missilecasing to be disrupted, relieving any build up of gas pressure andthereby preventing an explosion.

Another aspect to be considered in the application of the connectionmeans of this invention to mitigation of slow cook off in rocket motorcasings is the thermal heating arising in the casing and surroundingstructure after the rocket has been fired and the propellant has beenconsumed. “Heat soak” effects occur whereby heat is transferred from thehotter parts to the cooler parts. The temperature responsive connector,being well insulated, would normally be one of the cooler components, soits temperature would be expected to continue to rise after propellantburn-out. Therefore there is the possibility that the connector maydisengage at some later stage in the missile flight causing the missileto break apart. Normally, this would be undesirable, and so theinsulation provided would need to be sufficient to ensure that this didnot happen. However, there are circumstances in which disengagement ofthis kind would be desirable. For example, with a multiple stage rocketmotor, once the rear part of the missile has performed its role it willonly contribute to the drag and in this situation, the heat flow intothe temperature responsive connector could be arranged to bring aboutthe disengagement of the component parts of the casing automatically atan appropriate point in flight.

Shape memory alloys may also be used in a way that affords a rupturingaction on a munitions casing or other component which is to bedisrupted. According to a second aspect of the present inventiontherefore, there is provided an overwound munitions casing incorporatingan annulus of a shape memory alloy which has been subjected to acombination of mechanical and thermal treatments and which has acomposition such that upon subsequent heating to a predeterminedtemperature, said annulus will contract radially inwardly and rupturethe said munitions casing.

The annulus may be formed from a solid ring of shape memory alloy oralternatively a plurality of windings of shape memory alloy in wireform. The advantage of the latter is that the wire may be wound directlyonto a casing, whereas a solid ring would have to be pre-shaped to fitthe surface to which it is to be fitted. Further, windings may beespecially useful if the casing has a waisted or tapered section or hasan irregular surface area, as the wire will automatically adapt to thecontour of the surface during the winding process. Thus, the SMA wirerupturing (device) provides a more versatile cutting tool than the fixedcollar.

The SMA is treated by stretching or expanding at a temperature below thepredetermined temperature, in order to impart the memory function intothe annulus. However in the case where the annulus is in the form ofwindings, the memory may be imparted by placing the wire under tensionduring the winding process at a load sufficient to impart memorydeformation to the wire, thus reducing the number of processing stepsrequired

The annulus may be produced from any suitable shape memory alloy and mayfor example be selected from Cu—Al—Zn, Cu—Al—Ni, Cu—Zn—Mn—Al,Cu—Ni—Al—Zn—Mn and Ti—Ni alloys. If in wire form the SMA must also beductile and capable of being drawn into a wire. The selection of theload or work applied to the solid ring or wire will depend upon thealloy selected and the strength of the material which forms the casingto be cut; the higher the load imparted on to the wire the greater thecompressive force that can be applied.

The SMA annulus is designed to contract in use upon heating to afford arupturing or cutting action for example in respect of an overwoundrocket motor where the rupturing device acts a mitigation device toprevent an explosion on slow cook-off. Alternatively the element couldbe a container which is filled with water or a fire dispersing material,wherein the annulus is applied so that when in the presence of a firethe container is cut, releasing the water or dispersing material todouse the fire,

In an alternative arrangement the rupturing device may be used in anactive system, such that heat is deliberately applied to the annulus tocause it to contract. A simple method of generating internal heat in theSMA wire could be achieved by resistive ohmic heating 99, which could beachieved by either direct application of a current to the SMA annulus orby inducing a current in the annulus to achieve heating. It will beclear to the skilled person that other heating means for both solid andwire annuli may be employed, such as external heating wires or a radiantheater. By careful control of the rate of heating and the total heatapplied the concomitant rate of contraction and total force provided bythe contraction of the annulus can also be controlled. This allows theuser to select the amount of damage or degree of rupturing to the casingthat is desired, ranging between merely distorting the component throughto actually cutting it open. In the situation where the annulus is beingused as a mitigation device it is desirable that the casing is at leastsplit by the action of the annulus so as to effect the necessary releaseof pressure.

Typically this arrangement may be suitable for any thin walled munitionscasing such as lightweight rocket motor tubes or for launch tubes suchas are used in man-portable rocket propelled weapons, eg. man-launchedanti-tank weapons.

If a contracting SMA wire is to be used to cut a case or tube, it may bedesirable to concentrate its effect over as short a length of casing aspossible. It will be appreciated that if a wire is wound directly on toa surface it may be difficult to achieve a thick narrow band ofmaterial, as the wire may have a tendency to spread. Therefore toconcentrate the load it may be desirable to wind the wire into a housingof substantially U shaped form, such that the wire is retained withinthe housing. The housing shape and more importantly the contact areabetween the housing and the casing to be cut will affect the pressureapplied by the contraction of the wire. The housing is not necessarilyrequired to extend right around the perimeter of the casing to be cut,such that a gap may be left in the housing, for ease of fitting on thecasing, however the gap should be sufficient such that as the SMAcontracts the gap never closes fully. This ensures that the SMA does nothave to devote any of the force it generates to unnecessarily drivingthe housing into hoop compression, as would be the case if the housingformed a continuous ring. It may further be desirable to incorporatenotches in walls of the housing in order to reduce its flexuralstiffness, the objective being to avoid the SMA performing unnecessarywork in bending the housing, allowing the radially exerted force to beconcentrated into cutting the casing.

A complication can arise if the casing is made of a high elongationalloy, such as certain aluminium alloys. The SMA may be able to exertsufficient force to cut the case, but the recovery strain achievable bythe SMA may be lower than the strain to failure of the alloy, such thatthe contracting SMA would form a deep circumferential groove in thecasing but would not necessarily cut it. One solution to this is toconcentrate the cutting action over only part of the circumference ofthe casing. This may be achieved by enlarging a portion of the SMAhousing by the use of lateral flanges around part of the circumference.The flanges, where used, will spread the load over a wider area of thecase. Therefore the cutting action will be concentrated on the remainderof the housing without a flange, thus increasing the cutting efficiency.The selection of the optimum length of “unflanged” housing is acompromise between two considerations. A short arc has the effect ofconcentrating the effect of the SMA into a short arc, but the cuttingmay not penetrate very deeply into the casing because the distancebetween the chord and the arc is small. Thus, as the radius of curvatureof the housing increases as it “bites” into the casing, so the radialforce it exerts decreases. This mitigates against the use of a veryshort unflanged length. It will be evident that for a slow cook offmitigation action, a crack running part way around the casing issufficient, provided the length of the crack exceeds a critical value,as the action of a subsequent pressure build-up is likely to cause thecrack to propagate around the circumference and afford the desiredpressure reduction.

The approach of using a wire is desirable where the motor tube isthinned (“waisted”) on its outer diameter, because with a solid ring itmight be impossible to achieve a sufficiently tight fit around the motorfor the subsequent cutting action to be effective. As overwinding withfibres is a common method of constructing rocket motor cases, it will beconvenient also to include SMA wire within the overwind.

The cutting action of a contracting annulus may be enhanced by theincorporation of a cuffing device. This device may comprise a metal orceramic spike, blade or sharpened edge 86, which may be mounted in aseparate housing 88 to retain and direct it. The cutting device isplaced between the annulus and the casing to be cut. Upon contraction ofthe annulus, the device will be forced radially inwards, cutting intothe casing to produce an opening. It will be readily appreciated by aperson skilled in the art as to the size of opening required to allowthe explosive to be mitigated in any particular munition. The size ofcutting device may then be selected to create the desired size ofopening. Further, it may also be desirable that the cuffing device, whennot in use, is in held a retracted position, such that it is not inpermanent direct contact with the casing to be cut. In this way, anyweakening or premature rupturing of the tube in normal service isavoided. This retraction of the cutter may be achieved by, for example,placing a sacrificial spacer or a bias means, such as a set of springsbetween the cuffing device and the casing. Alternatively the cuffingdevice may be retained by pins, or adhesive, which can be sheared, orcaused to fail by other means, by the action of the contracting SMA.

For some types of casing the action of a contracting band on its outsidemay cause it to buckle before it cracks. Which mode of failure (i.e.cracklng or buckling) occurs first depends on the wall thickness of thecasing, its diameter and the modulus and strength of the material fromwhich it is constructed. If the casing is laminated or of a compositeconstruction, this may also affect the failure mode. In the event ofbuckling occurring, it is possible and desirable to concentrate thebuckling action into one deep fold, by any one of the aforementionedtechniques. The sharp curvature at the bottom of the fold may then besufficient to cause the casing to crack. In this situation the type ofhousing is not as important as it is for cutting and so the SMA may beapplied as a broad band.

The SMA based mitigation devices described up to this point are passivein that they respond to the external heating threat without the need forsensors to detect the threat or energy sources to trigger the SMA. Whenused in this way they have the merits of simplicity and obviate the needfor additional energetic materials, which introduce fresh hazards, orpower sources such as batteries that introduce lifing and maintenanceissues. However, all the configurations described can be converted intoactive mitigation devices by the use of additional sensors and powersources. In the case of slow and fast cook-off, it might also bedesirable to incorporate some kind of electronic logic circuit in orderto anticipate the event and activate the SMA accordingly.

Therefore in one embodiment of the invention the SMA device will have aheating means, such as an electrical supply connected. There may also beprovided a heat sensing means and a manual activation capability suchthat one could actively choose to disengage or rupture the munition, asfor example when a rocket motor is jammed in an aeroplane or helicopterlaunch tube, or if the need arose to break up a rocket in mid flight.The SMA device could still function in the normal passive mode, that iswhen its surroundings reach the SMA transition temperature, but theactive mitigation would form an additional option.

The invention will now be further described with reference to theaccompanying drawings and example in which:—

FIG. 1 is a partial cross section through a connection device accordingto the invention having an internal thread in conjunction with twosections of a rocket motor casing which possess complementary externalthreads;

FIG. 2 is a partial cross section through a connection device accordingto the invention having two or more lugs or alternatively twoinwardly-projecting lips at the extremities of the annulus, and showsthe device in use to join together two pipes or columns which possesscomplementary recesses;

FIG. 3 is a partial cross section through a connector according to theinvention, where one pipe to be joined has an internal thread and asecond pipe has a complementary external thread;

FIGS. 4 a and 4 b are longitudinal sections of part of an overwoundrocket motor casing where part of the overwinding comprises an SMA wireoverwind (4 a is prior to and 4 b is the result after activation of theSMA wire);

FIG. 5 is a graph showing a typical stress versus strain plot for an SMAwire material;

FIG. 6 shows a partially flanged housing, for containing the wirewindings, in elevation, mounted on a munition casing (shown in crosssection), prior to activation;

FIG. 7 is a cross section through the housing of FIG. 6;

FIG. 8 shows the housing of FIG. 6, after activation.

FIG. 9 is a drawing of one mode of rupturing of the casing of amunition, by buckling and cracking due to the action of an annulus ofSMA.

FIG. 10 is a section through housing 85 taken on a plane that is radialwith respect to the munition casing 81. The housing (85) is seen tocontain a plurality of SMA wire windings (83). The cuffing action of acontracting annulus (82) may be enhanced by the incorporation of acuffing device (84) as shown in FIG. 10. This device may comprise ametal or ceramic spike, blade or sharpened edge (86). which may bemounted in a separate housing (88) to retain and direct it. Further, itmay be desirable that the cuffing device (86). when not in use, is heldin a retracted position such that it is not in permanent direct contactwith the casing (81) to be cut. In this way, any weakening or prematurerupturing of the tube in normal service is avoided. This retraction ofthe cuffer (86) may be achieved by, for example, placing a sacrificialspacer (87) between the cuffing device and the casing.

FIG. 11 is a section through housing 95 taken on a plane that is radialwith respect to the munition casing 91. The housing 95 is seen tocontain a plurality of SMA wire windings 93. In an alternativearrangement the rupturing device 90 may be used in an active system,such that heat is deliberately applied to the annulus 92 to cause it tocontract. A simple method of generating internal heat in the SMA wire 93could be achieved by resistive ohmic heating, which could be achieved byeither direct application of a current 99 to the SMA annulus or byinducing a current (not shown) in the annulus to achieve heating.

In the embodiment shown in FIG. 1 two sections of a rocket motor caseare shown at (1, 1 a). Each has a threaded portion (2, 2 a) on itsoutside face. The connection means (4) is an extended annulus of shapememory alloy, having an internal thread (3) which is complementary toexternal threads (2, 2 a) on the two sections of rocket motor casing (1,1 a). The rocket propellant charge (not shown), will occupy the volumeenclosed by the casing. The interface (11) between the two rocket motorsections (1, 1 a) is reinforced by respective stepped shoulders (7, 7 a)formed on the outside faces of the casing sections. A metal insert (6),which can be of SMA or any material capable of providing mechanicalsupport, is seated against shoulders (7, 7 a). Insert (6) may beindependent of the connection means (4) or integral with it. To ensure agas tight seal during normal operation two o-ring seals (10, 10 a) arelocated in the channels (5, 5 a) in the respective casing sections.“Memory” will have been imparted into the SMA during a previous formingoperation. For example it may have been passed through a tapered die toreduce its diameter or compressed radially by the application ofexternal pressure.

When subjected to a thermal hazard such that a predetermined temperatureis reached, the connection means (4) is arranged to deform, bycontraction along its axis plane, causing the internal thread (3) of theconnection means to move against and to break the external threads (2, 2a) of the two rocket motor sections as a consequence of which the tworocket motor sections will separate and allow the pressure inside therocket motor to vent. In an alternative arrangement the connector (4)simply expands so as to disengage the threads 3 and 2, 2 a respectively,again allowing the motor sections to separate, but in practice it islikely that both mechanisms will operate simultaneously. It will bereadily appreciated by the skilled person that the connector 4 couldpossess an external thread, and that it could be located instead on theinside of the two rocket motor sections (1, 1 a) which in turn wouldpossess complementary internal threads. In this arrangement theconnector is designed, on being heated, to contract radially withconcomitant expansion in the axial plane, thus again affordingdisengagement of the threaded portions and separation of the two rocketmotor sections.

In the embodiment shown in FIG. 2, two members (14, 14 a) which may becylindrical or of other section and either solid or hollow are to bejoined at the interface (17). The connection means (13) is a sleeve oflike section to the members having annular projections (16, 16 a) whichlocate into respective recesses (15, 15 a) formed in the members to bejoined towards the respective ends thereof. (It will be appreciated thatthe projections and recesses may equally well be continuous, ie. anupstanding annulus and an annular groove or channel respectively andalso that the locations of the recess(es) and projection(s) could bereversed). The connected unit 12 may comprise a part of an oil rig orother structure which it is desired to disassemble remotely at somefuture time. The connecting sleeve 13 is made from an SMA which isshrunken onto the members and is so chosen that on heating to apredetermined temperature it will expand sufficiently to becomedisengaged from the members (14, 14 a) thus allowing them to beseparated. It will be readily appreciated by the skilled person that theconnecting sleeve can be activated by cooling, which would be moreappropriate for any structure that has to meet a fire hazard duringservice.

In the embodiment of FIG. 3 two cylinders 18, 19 (which may be eithersolid or tubular) are to be joined. In this case the connection means isintegrated with the members to be joined. Thus cylinder (18) has aninternal threaded section (20), while cylinder (19) has a complementaryexternal threaded portion (21). The two cylinders are brought intoengagement by screwing them together. At least one cylinder thread (20,21) is manufactured from a shape memory alloy and may be an inset oralternatively one or both of the cylinders may be entirely manufacturedfrom a shape memory alloy. When the connection means is either heated orcooled to a predetermined temperature (as desired), at least oneoperative part of the connection means (either 20 or 21) is arranged todeform, by either contraction or expansion radially and/or along itsaxis, causing the threads to disengage and/or be sheared off, as aconsequence of which the two cylinders will disengage and be separated.As a variant on this arrangement, both co-operative parts of theconnection means may be formed from SMAs and be arranged such that, uponheating or cooling, one of the threads expands radially and the othercontracts radially, to more readily afford separation of the two.

In the embodiment of FIG. 4 a there is shown an SMA cutting device. Asection of thin walled (typically aluminium alloy) rocket motor case(22) is shown, which has a series of windings of (stretched) SMA wire(24) around one part of the rocket motor case (alternatively (24) couldbe a solid annulus or collar formed from an SMA). The motor caseincluding the SMA winding or collar (24) is then overwound with areinforcing fibre (23), which may be an aramid (e.g. Kevlar) or carbonfibre. When the SMA wire (24) is subjected to heating through thetransition temperature range of the SMA alloy, the wire (24) willcontract along its length and hence the winding will contract radiallyeither simply cutting the rocket motor case (FIG. 4 b) or causing it tobuckle and crack.

In this way the pressure build-up in the casing as a result of asubsequent cook-off event is avoided. Alternatively instead of usingwire, a solid collar or ring of SMA can be used. If the SMA ispreviously expanded at the appropriate temperature for imparting“memory”, it will contract when heated through the transitiontemperature range for the specific SMA being used.

The stress strain curve of FIG. 5 shows that as a load is applied to anSMA wire material, ie a tension force is applied, the stress and strainboth increase. A strain induced phase transition occurs in region (30).The application of a further load past point 32 and further up line 33imparts a ‘memory’ or ‘work’ into the alloy, such that upon eventualrelease of the load, the material will contract along line 31. Thereforewhen winding the wire onto a casing, one can either apply a loadsufficient to take the SMA past point 32, or alternatively the wire canbe pretensioned past point 32 and then wound under a reduced tension.

In an embodiment of the wire winding arrangement of the invention shownin FIG. 6, a housing (40) to contain the SMA wire (41) is shown asviewed from along the axis of the munition and located around the casingof the munition (45) (shown in section). The housing may extend eitherpartially (not shown) or substantially fully around the casing. Byarranging that the housing extends only partially around the casing, itcan be ensured that the gap (52) between the ends of the housing doesnot fully close upon contraction of the wire (41). To further reduce theflexural stiffness of the housing, a series of notches (53) may beincorporated in the walls thereof, to allow the housing to bend andtherefore curve more easily around the perimeter of the casing duringthe contraction of the wire, such that substantially all of the forcebeing exerted by the wire is directed towards rupturing the case.

A section through the housing taken on a plane that is radial withrespect to the munition casing is shown in FIG. 7 and the housing isseen to contain a plurality of SMA wire windings (41). The housingcomprises a channel member and optionally flanges (44) which extendlaterally of the channel member, as shown also in FIG. 7. The externalshape of the housing is selected to give an effective cutting action.Thus in FIG. 7 the housing (40) is shown as being substantiallysquare/rectangular in cross section with walls (42) to retain the wire(41) and a base (43) which is seated against the casing of the munition(45). For ease of winding the wire, the internal profile of the base ofthe housing may be rounded in cross section, such as typically a U-shapeso as to give a smooth profile at the junction of the walls (42) and thebase (43). As the wire contracts the greatest cutting force is exertedeither across the region of the gap (52) between parts of the housing,where the wire (41) comes into direct contact with the casing (45), orin the alternative arrangement where the housing is a combination offlanged (61, 62) and unflanged (63) regions and the cutting occurs inthe unflanged (63) region.

FIG. 8 shows the inward displacement of the non-flanged region of theembodiment of FIG. 6 after activation of the SMA. The gap (52) in thisarrangement may be reduced in length, such that only a minimum amount ofwire (41) is in contact with the case, so that the cutting force is thenconcentrated instead across the non-flanged region (63), as shown inFIG. 8.

In the embodiment of FIG. 9, there is shown one of the rupture failuremechanisms, where a wire is located in a housing (not shown), or isapplied directly to the casing (45) (as shown in FIG. 4) and causes thecasing to buckle or crumple. The failure point, or crack (71) occurs onthe inside surface (72) of the casing (45) which is the point ofgreatest tensile stress. The failure point will then propagate radiallyoutwards to the outside of the case (73) to produce a completeperforation of the case. As further load is applied to the perforation,the crack will tend to elongate along the length of the casing. In aslow cook-off incident, once the crack has perforated the case, thebuilt up pressure from the energetic material (not shown) as itdegrades, will assist in further elongating the perforation.

In certain situations the ‘heat soak’ effect described previously may beutilised to cause the automatic rupturing of the rocket motor case at anappropriate point in its flight.

Alternatively, the use of an SMA collar or wire overwinding could beapplied to a lightweight launch tube for missiles and hence thecomponent 22 in FIG. 4 could be such a launch tube instead of a rocketmotor case.

EXAMPLE

A length of Ti—Ni wire 0.125 mm in diameter, was stretched by 9% toimpart a memory and was then cut into 1 metre lengths. Separate lengthswere hung vertically with weights of 0.55 Kg (corresponding to a tensilestress of 448 MPa in the wire), 0.75 kg (corresponding to 611 MPa) and1.00 Kg (corresponding to 815 MPa) suspended from them. The wires wereheated by the application of a current and the resulting recoverycompressive strain (under load) measured. Respective length contractionscorresponding to recovery strains of 7.1%, 5.9% and 4.9% were recorded,showing that considerable displacements can be achieved even when thestress opposing the contraction of the wire is as high as 815 MPa.

1. A munitions casing comprising an annulus of a shape memory alloydisposed around said casing which shape memory alloy has been subjectedto a combination of mechanical and thermal treatments so as to impart amemory wherein upon subsequent heating to a predetermined temperature,said memory causes said annulus to contract radially inwardly andrupture the said munitions casing.
 2. The casing as claimed in claim 1,wherein the shape memory alloy is selected from Cu—Al—Zn, Cu—Al—Ni,Cu—Ni—Al—Zn—Mn, Cu—Zn—Al—Mn and Ti—Ni alloys.
 3. The casing as claimedin claim 1 wherein the annulus is a wire winding and is wound within ahousing which is located around the casing.
 4. The casing as claimed inclaim 3 wherein the housing extends wholly or partly around theperimeter of the monition casing.
 5. The A casing as claimed in eitherclaim 3, wherein the housing is U-shaped or rectangular in crosssection.
 6. The A casing as claimed in claim 5, wherein part of thelength of the housing is provided with a flange which extends laterallyon each side of the base of the housing.
 7. The casing as claimed inclaim 3, wherein the walls of the housing are cut to provide reducedflexural stiffness.
 8. A method of using a munitions casing as clamed inclaim 1 comprising locating the annulus, around the outer surface of themunitions casing and arranging for an internal heater to be applied tosaid at least one annulus, wherein the internal heater is capable ofproviding subsequent heating to the predetermined temperature so as tocause the annulus to rupture the munitions casing.
 9. The casing asclaimed in claim 1, wherein the shape memory alloy has a transitiontemperature range which lies in the range of 80°C -150°C.
 10. The casingas clamed in claim 1, wherein the annulus is comprised of a plurality ofwindings of shape memory alloy in wire form.
 11. The casing as claimedin claim 1 which is a casing for a shell, bomb, torpedo, missile orrocket motor.
 12. The casing as claimed in claim 11, wherein themunitions casing is an overwound munition.
 13. The casing as claimed inclaim 11 containing an energetic material.
 14. The casing as claimed inclaims 13 wherein the energetic material is propellant or highexplosive.
 15. The casing as claimed in claim 1, which forms part of alaunch tube assembly.
 16. A method of manufacturing a munitions casingas claimed in claim 1, wherein the annulus of the shape memory alloy isi) subjected to a combination of mechanical and thermal treatments andis selected to have a composition such that, when installed around themunitions casing and subjected to subsequent heating to a predeterminedtemperature, said annulus will contract radially inwardly and rupturethe said munitions casing; and ii) installing the pretreated annulus ofthe shape memory alloy around the munitions casing.
 17. The method ofclaim 16 wherein the shape memory alloy that forms the annulus isstretched or expanded at a temperature below the predeterminedtemperature prior to fitting on the munitions casing.
 18. The casing asclaimed in claim 1, wherein the annulus is comprised of a solid ring ofshape memory alloy.